The present invention generally relates to the field of surface enhanced Raman spectroscopy, and more particularly, to a method for bonding metallic films to a glass substrate to create a structure suitable for use in surface enhanced Raman spectroscopy.
In the 1970s, it was discovered that Raman scattering from molecules of an analyte of interest adsorbed on noble metals such as silver, copper, and gold when irradiated with optical energy can be enhanced by as much as 106 to 107 compared to merely irradiating the analyte. This phenomenon is known as surface enhanced Raman spectroscopy (SERS). A SERS structure generally includes a metal layer formed on a substrate and is used to detect the presence of an analyte by examining the emissions from the substrate when irradiated with optical energy. SERS emissions, or spectra, have been used to detect and identify trace organics and as a detection method in gas chromatography, liquid chromatography, and thin layer chromatography. Electrochemical SERS and SERS of chemically modified surfaces have been used to detect aromatic compounds and chlorinated hydrocarbons and other organic contaminants of environmental concern in the ppm range.
The generation of SERS spectra for molecules adsorbed on metal surfaces requires that the metal surface be roughened. The most common types of SERS substrates include electrodes, colloidal solutions, island films prepared by vacuum deposition of metal and metal-covered surfaces having submicrometer structures such as micro spheres, monolithic posts, etc.
The SERS phenomenon is wavelength dependent. In general, greater enhancements are observed using near infrared (IR) excitation. However, water strongly absorbs near IR energy. Consequently, the SERS emissions obtained for aqueous samples tend to be greatly attenuated. The more water through which the excitation energy must penetrate, the greater is the attenuation. Therefore, there is a strong motivation to develop a SERS system for detecting analytes of interest in an aqueous environment that minimizes attenuation of the IR excitation signal.
The use of SERS techniques in an aqueous environment strongly suggests the need for a method for fabricating a thin, transparent, and durable metal layer on an optically transparent substrate, such as glass, to create a composite transparent structure. A transparent substrate would be desirable so that the interface at the aqueous environment adjacent to the transparent composite structure could be illuminated with minimal attenuation of excitation energy through water. If an analyte of interest is present, SERS spectra will be emitted from the surface of the composite transparent structure at the interface and directed back through the structure to an optical receiver.
One type of composite transparent structure suitable for use in SERS includes silver films that are vapor deposited onto an optically transparent substrate made of silicon oxide (SiO2), such as glass. However, metals such as gold and silver do not adhere well to glass. Significant exposure to water damages the metal films, thereby limiting their applicability in aqueous environments. Therefore, a need exists for a method for securely forming metal films onto a transparent substrate that produces a durable structure suitable for generating SERS spectra. A further need exits for manufacturing a SERS structure that is durable in aqueous environments.
The present invention provides a SERS structure for detecting an analyte of interest that exhibits high SERS activity and good adhesion to glass. Such substrates may be used in aqueous environments for extended periods of time. The SERS substrate includes a glass substrate having a specially roughened surface on which an adhesion layer is formed. A discontinuous noble metal layer is formed, as for example, by vapor deposition on the adhesion layer. A thiol coating covers and protects the metal layer from chemical contamination, thereby extending the lifetime of the structure to months when exposed to an aqueous environment. The type of thiol of which the coating consists is selected to have an affinity for the analyte. The roughened surface provides the structure with a good SERS response.
A SERS structure may also be manufactured by subjecting a smooth surface of a transparent glass substrate to a silanization agent to create an adhesion layer, and then forming a metal layer on the adhesion layer. Next, the metal layer is subjected to an electrochemical etch which transforms the metal layer into electrically isolated metal islands. Finally, the metal islands are subjected to a thiol solution which forms a self-assembled monolayer that protects the islands and provides the SERS structure with sensitivity for particular agents of interest. The metal layer preferably consists essentially of a metal selected from the group that includes copper, gold, and silver. In the preferred embodiment, the silanization agent is a 1:10 mixture by volume of (3-mercaptopropyl) trimethoxysilane in ethanol.
The metal layer of the SERS substrate embodied in the present invention is very durable compared to those of prior art structures. Therefore, the present invention should find wide application in the application of SERS techniques for detecting organic contaminants in air and aqueous environments, and for detecting metallic and anionic contaminants in water.
These and other advantages of the invention will become more apparent upon review of the accompanying drawings and specification, including the claims.